Recently, secondary batteries have been used for various applications, for example, cell phones, personal computers, power tools, HEVs, and PEVs, and alkaline storage batteries have come to be used for these applications. Among these applications, especially in alkaline storage batteries used for consumer applications such as cell phones, personal computers, and power tools, a complete charge-discharge control method wherein complete charging-discharging is performed is applied.
It is known that addition of zinc to a positive electrode mixture having nickel hydroxide as a main positive electrode active material is generally effective to improve the cycle life of an alkaline storage battery wherein the complete charging-discharging is performed. In this case, zinc is generally added to the positive electrode mixture so as to be approximately 15% by mass with respect to the mass of nickel as the positive electrode active material. The addition of such amount of zinc makes it possible to inhibit swelling of the positive electrode active material associated with the charge-discharge cycle and to extend the life of the battery.
Meanwhile, in alkaline storage batteries used for applications relating to vehicles such as HEVs and PEVs, a partial charge-discharge control (ΔSOC-pulse Cycle) method is widely used for performing partial charging-discharging. For example, JP-A-2005-108610 discloses an alkaline storage battery suitable for the partial charge-discharge control method for performing partial charging-discharging.
On the other hand, in such alkaline storage batteries, with which partial charging-discharging is performed, used for such applications relating to vehicles as HEVs and PEVs, it is known that a memory effect is a main factor for reduction of the partial charge-discharge cycle life. Accordingly, it has been revealed that in the alkaline storage batteries used for such applications, the addition of zinc to the positive electrode mixture by approximately 15% by mass with respect to the mass of nickel as the positive electrode active material does not improve the charge-discharge cycle life, but adversely affect the battery with the added zinc serving as a resistance component.
Furthermore, in the alkaline storage battery disclosed in JP-A-2005-108610, the concentration of an alkaline electrolyte is as high as 7 mol/L (7 N), and the amount of lithium (Li) contained in the alkaline electrolyte is as small as 0.05 mol/L (0.05 N). As a result, the memory effect is not improved, whereby a charge-discharge cycle characteristic is not improved.
Therefore, as a result of extensive studies on the reasons, the inventors of the present invention have found that, for alkaline storage batteries with which partial charging-discharging is performed, reducing the addition amount of zinc contained in the positive electrode mixture inhibits the memory effect so that the batteries have a long life. Furthermore, they have also found that making the concentration of the alkaline electrolyte less than 7 mol/L (7 N) and increasing the amount of lithium (Li) contained in the alkaline electrolyte inhibit the memory effect increases the life of the batteries.